I did a direct A/B comparison of my Peavey cables with Monster cables and I noticed nothing. The only cables the make an audible difference to my ears are speaker cables. A good quality speaker cable will have more punch and clarity than a generic speaker cable.

I've never found a cable that "added" to the sound, but I have certainly found cables that detracted from it. I usually just buy well made cables that won't get easily broken if stepped on ect.... I am not so worried about sound

I noticed a difference when I switched from no names and Fender cables to George L's. More clarity and less loss of high end. I use the thinner ones for my pedal board and the standard ones for instrument cables.

I really like Planet Waves Cables. They are made seemingly very well and they produce a good sound. I have had Monster Cables in the past, and the 21' "Rock" cable that I bough clicks when you move around and aren't playing. It's quite annoying. I am currently in the process of changing everything over to Planet Waves cables. My other guitarist did and he really likes the sound that he is getting, and they seem to take abuse well on stage.

Guitar cables... I use the mid level ones that are built solid to last... not for sound. Some I've had for years... but for my rack, it's an entirely different story. By accident when installing new rack gear, I switched a cheapo patch cable connecting my Triaxis to my 2:90 with a Monster patch cable. To my amazement, a bothersome hiss which I always thought was inherent with this pre amp disappeared. I switched back to the cheapo to confirm. Good cables do make a significant difference in rack equipment which generates a ton of RF...